They met by chance one night at a party in Tottenham, north London. The man she would come to know as Bob Robinson was standing on his own. Jenny (not her real name), a 24-year-old who had come to the capital to find work, was intrigued by the slim man with the endearing smile, who was slightly older than her.

They fell easily into conversation and before long, Jenny was smitten. The love she felt for him rolls easily off her tongue. He was, she says, "polite, considerate, very romantic, attentive, charismatic". He smiled a lot and was non-judgmental. And he was cute.

"I thought I had found my Mr Right. He was very charming and I thought I could take him to meet my parents," she says.

They had an 18-month relationship and one of his characteristics struck her in particular: "I thought he had a high moral code."

But now she feels very different about him. It turns out that there was a lot more to Bob Robinson than his impassioned campaigning and shoulder-length hair, which gave every impression of a rebel with many causes.

He was, in fact, the opposite. Bob Lambert today admits he was an undercover police officer who had created the fictional persona of Bob Robinson to spy on political activists.

The special branch officer was one of a group of police spies in a covert unit who have been infiltrating and disrupting the activities of political campaign groups across Britain for decades.

Jenny and others only discovered his true identity more than 20 years after they first met him. The discovery has left Jenny feeling that he deceived her about the bedrock of any relationship – his identity. She is very hurt that he duped her about who he was. "I was cruelly tricked and it has made me very angry. I feel violated," she said.

As she was trying to persuade him to set up home and have a family together, he was resisting, claiming he had to flee abroad as he was being pursued by special branch because he was a dangerous radical activist.

The sorry episode has left her wondering if he loved her at all. Today, Lambert admits that "as part of my alter ego's cover story, I had a relationship with 'Jenny', to whom I owe an unreserved apology".

So far, seven undercover police officers who infiltrated political groups have been exposed – and most have admitted or have been accused of sleeping with activists they were spying on. They have faced claims that they did so to glean intelligence about the activists and the protests they were organising. A growing number of women say they have suffered terrible trauma and damage from the betrayal of having a relationship with a person they later found out was a fake.

Police chiefs claim that undercover officers are forbidden from having sex with their targets "under any circumstances" as it is "unacceptable and unprofessional". But Pete Black, an undercover officer from the same unit who infiltrated anti-racist groups in the 1990s, said sex was widely used as a technique to blend in and gather intelligence. He said there was an informal code in the unit that the spies should not fall in love with the women – or allow the women to fall in love with them.

An investigation by the Guardian has shown that Lambert was no ordinary police spy. His skills of deception would earn him legendary status in the elite ranks of the covert unit known as the special demonstration squad (SDS). "He did what is hands down regarded as the best tour of duty ever," said Black.

Lambert admits that in the 1980s, he "first built a reputation as a committed member of London Greenpeace, a peaceful campaigning group [on environmental issues]". He did so "as part of my cover story" to "gain the necessary credibility to become involved in serious crime".

His aim was to penetrate the Animal Liberation Front (ALF), which he says was "then engaged in incendiary device and explosive device campaigns against targets in the vivisection, meat and fur trades".

In the 1990s, he drew on the techniques he had learned undercover to become the head of operations in the covert unit, running a network of spies.

It was May 1987 when Jenny met Bob. Very quickly they were spending most of their free time together. Bob said he was a gardener, doing cash-in-hand jobs in well-heeled places such as Hampstead. He told her that he was also earning a living by driving a minicab, although he was touting illegally for customers.

But politics was really his thing, he said. He told her how he was deeply involved in campaigning for animal rights and the environment.

Bob confided that he was heavily active in the ALF. But she was not interested. "He was always asking me to go to meetings. He introduced me to lots of activists. I did not realise what the ALF was."

But why did Lambert have a relationship with Jenny when she had never been an activist ? "I have no idea. It's a great mystery," she says.

It seems from his admission today that he was using her as his girlfriend so that he could portray himself as a fully rounded person with a private life to the rest of his political and social circle. Activists, eternally on their guard against police spies, are suspicious of people who, for example, turn up at their meetings out of the blue without any discernible evidence of friends or a family. Taking her along to the pub or parties with other activists was a neat way of deflecting those suspicions.

Jenny was working at the time as an administrative assistant at the state-owned Central Electricity Generating Board. But she kept quiet about her job as she feared the activists would take against her because the CEGB was running nuclear power stations.

She was keen to develop her career and have a family. She lived in an east London house with eight other friends, but none of them were politically active, other than having a general antipathy to Margaret Thatcher's government.

They spent most nights together at her house, although he lived in what she called a "grotty flat above a barber's" in Hackney. He had a "single man's room with a shared kitchen" but with very little in it. "He claimed to be not interested in possessions," she said.

A few months into their relationship came the episode that was to seal Lambert's reputation as one of the best undercover operatives the SDS had ever had.

In the summer of 1987, Lambert had been undercover for three years and had worked his way into the inner recesses of the animal rights movement. The Animal Liberation Front operated through a tightly organised underground network of small cells of activists, making it difficult for spies to get among them. Police chiefs were on the hunt for sorely needed intelligence after three incendiary bomb attacks on Debenhams shops in Harrow, Luton and Romford. Activists had planted the bombs because the shops were selling fur products. The attacks had reputedly caused millions of pounds' worth of damage.

Lambert identified the perpetrators to his handlers. The intelligence was so precise that the police caught them red-handed. The Old Bailey heard how police raided a flat in Tottenham and found two activists sitting at a table covered with dismantled alarm clocks, bulbs and electrical equipment for making four more firebombs.

The prosecution told the court that Andrew Clarke, then 25, and Geoff Shepherd, then 31, were wearing gloves to conceal their fingerprints. The bombs were made in large matchboxes, with a warning: "Do not touch. Ring police. Animal Liberation Front." Shepherd was jailed for four years and four months, and Clarke for more than three years.

But his feat also went down in SDS legend because Lambert had skilfully disguised that he was the source of the tip-off, managing to throw the suspicions on to others within the small ring of activists who knew about the attacks. So well had he retained the trust of the activists that Jenny remembers that he went, with her, to visit one of the accused in jail while they were awaiting the trial.

Jenny remembers that after the arrests, Bob would often say that special branch was hot on his and other activists' trails. There was, he says, a "big crisis" because the animal rights campaigners suspected that there was an informer in their midst.

A bizarre incident happened at about that time. By 1988, Jenny had moved into a Hackney flat with two others, who were not politically active. One day, special branch detectives raided Jenny's home, letting slip that they were "looking for Bob". He was not there. She remembers that one of the detectives picked up a pair of shoes and asked who owned them. They belonged to Jenny. The raid, the Guardian understands, was orchestrated by police to bolster Lambert's cover story.

After more than a year together, Jenny felt that Bob had given her the right signals that he was interested in having children with her. He had been to see her parents three times. But when she broached the question, he said no, upsetting her hugely. She wrote in her diary that it was a black day. "I remember crying a lot that day. I was just so shocked."

Soon afterwards, she says, Bob began to tell her that he would have to go on the run abroad to escape the special branch. Over the last few months of 1988, they discussed what to do. She said she wanted to go with him, but he said she should not.

According to Jenny, he argued that she should not waste her life on the run, constantly looking over her shoulder, and that she deserved better – a rewarding career and a family. "He said he was not good enough for me."

He left his flat and stayed for a couple of weeks in what she called a "safe house" with one of her friends in London. She remembers meeting him once there: there was "still a lot of electricity between us".

In December 1988, Bob and Jenny spent a week alone together in a friend's house in Dorset to say goodbye. "I was heartbroken. Even when he left, I could not imagine that it had finished because we loved each other so much. I wanted to go on the run with him. I was prepared to do that for him."

But his sacrifice in not taking her with him made her admire him even more.

He said he was going to Spain. In early 1989, she received a long letter from him in Valencia, saying he was not coming back but raising the possibility that she could join him there. "Even then I could not believe it," she says. It was the last she heard from him.

The drawn-out goodbye was a ruse. His trip to Spain and the postmark on that letter was genuine, but the reasons were not. Bob's undercover tour was ending and he needed to leave the activists without arousing suspicions. Using standard tradecraft, he had created the perception of a convincing reason for his departure – that special branch were after him. The Spanish bolthole was far enough away to deter activists from going to see him, and avoid the risk of their bumping into him.